Ceramics are superior to metals in heat resistance, corrosion resistance, wear resistance, etc., and are the materials to be hoped. Examples of such ceramics include alumina, silicon nitride, silicon carbide, partially stabilized zirconia, and the like. It is known to impart novel functions in addition to the native properties of the sintered body of the above material by forming the membrane of a foreign ceramic on the surface of the sintered body.
For example, it is known to coat the surface of a silicon nitride sintered body with alumina by CVD. Silicon nitride itself is excellent in strength and toughness, but it tends to react with iron. Therefore, the uses as cutting tools for steel materials are restricted, whereas alumina coating prevents the reaction of silicon nitride with iron resulting to extend the life of the cutting tools.
Besides, it is also known to increase the resistance to the damage during using it by the thermal spraying of zirconia onto the surface of a silicon nitride sintered body (J. L. Schienle and J. Smyth; High Temperature Coating Study to Reduce Contact Stress Damage of Ceramics, Proc. Twenty-Third Automotive Technology Development Contractors' Coordination Meeting, (1985), p.249).
As mentioned above, as the method of forming a membrane of foreign ceramic on the surface of a ceramic, there are the above methods according to CVD or PVD, and furthermore the method of applying the liquid of metal polymer onto the surface of a sintered body followed by sintering, etc. A dense membrane can be formed by coating the surface of a sintered body by CVD. However, there are the problems that the film-forming speed is small, and the coating takes a lot of time. Furthermore, it is difficult to form an uniform membrane on complex-shaped articles, and the application of this method to big articles is also difficult. There are similar problems in PVD.
Another method of forming ceramic membrane is comprised of coating the surface of a sintered body with the suspension of ceramic powder, the solution of hydroxide, the liquid, paste or solution of metal polymer or the like by spraying, brush coating, immersing or the like, drying, pyrolyzing, and then sintering. Besides, the method of thermal spraying of ceramic powder is also known. The restrictions of these methods to the shape and the size are less than CVD, PVD. However, there is a common disadvantage in these methods. That is, the produced membrane contains pores, and in most cases, atmospheric gas contacts the substrate of the sintered body through the pores, and for example, some objects of the coating, such as the improvement in oxidation resistance, are not achieved.
A method of converting a membrane with much pores to a dense membrane with little pores by the hot isostatic pressing (HIP) is proposed in order to solve the above problem (H. Kuribayashi et al; Ceramic Bulletin, vol.65, No.9, (1986), p1306). This method comprises forming a ceramic membrane on the surface of stainless steel by plasma spraying, sealing it with a vessel of pyrex glass, using argon gas as the pressurizing medium, and subjecting it to HIP. Thus, dense alumina, zirconia and titanium carbide membranes were formed on the surface of stainless steel.
A flow sheet of the method of conducting the above HIP treatment is shown in FIG. 4. Respective processes are explained more concretely. First, a known molding method, such as uniaxial pressing, CIP, extrusion, slurry casting or injection molding, is applied to the raw powder of a ceramic sintered body to obtain a molded body, and various sintering methods are applied to the molded body to obtain a sintered body. As the sintering method, for example, the low pressure sintering method wherein the sintering is conducted under vacuum, ordinary pressure or a pressure of less than 10 atm. is known. In this method, since pores usually remain, it is also known to densify it by further conducting HIP treatment. The method of producing a porous sintered body by preparing a molded body containing metal silicon powder and nitriding by heating in a nitrogen atmosphere to obtain a porous sintered body is also known. The method of obtaining a dense sintered body by sealing a porous body such as a molded body or a porous sintered body with a capsule such as one made of glass, conducting HIP treatment, and removing the capsule is also known. It is also known to obtain a dense sintered body by applying a precursor for a capsule capable of conversion into capsule by heating such as glass powder or metal polymer onto the surface of a porous body by spraying, brush coating or immersing, forming a gas-impermeable membrane by drying, thermal decomposition and sintering, using it as the capsule, conducting HIP treatment, and then removing the capsule. Moreover, the hot pressing (HP) method comprising placing a porous body in a carbon die and uniaxially pressing with heating is also known.
A precursor for a ceramic membrane such as the suspension of ceramic powder, the solution of hydroxide or the liquid, paste or solution of metal polymer is applied to the sintered body thus obtained by means of spraying, brush coating, immersing or the like, and a ceramic membrane formed by conducting drying, thermal decomposition and sintering. The ceramic membrane contains a considerable quantity of pores, though the quantity varies.
The ceramic sintered body having the porous ceramic membrane is sealed with a capsule of glass or refractory metal (tantalium, niobium, etc.), HIP is conducted, and the capsule is removed. Thus, a ceramic sintered body having a dense ceramic membrane is obtained.
As mentioned above, the conventional method shown in FIG. 4 is divided into the process for producing the porous body, the process for producing the sintered body, the process for ceramic coating and the process for densifying the ceramic membrane, and therefore, it has a problem that the total processes are extremely long. This invention has been made in order to solve the above problem, and an object thereof is to obtain a method of producing a ceramic sintered body having a dense membrane of a ceramic which is different from the ceramic composing the substrate of the sintered body through short processes.